1. Technical Field
The present invention relates to an imaging apparatus having an auto-focus function.
2. Related Art
Recently, as the high resolution of a solid imaging element such as a CMOS (Charge Coupled Device image sensor or a CMOS (Complementary Metal Oxide Semiconductor image sensor has been attained, a demand for information devices having a photographing function such as a digital still camera, a digital video camera, a cellular phone (e.g., a smart phone), and a personal digital assistant (PDA) is rapidly increasing. Meanwhile, information appliances having an imaging function as described above are referred to as imaging apparatuses.
The imaging apparatuses employ a contrast auto-focus (AF) method or a phase difference AF method as a focusing control method for focusing on a main object.
The contrast AF method is a method for acquiring a picked-up image contrast, obtained in each driving step, as an evaluated value while moving a focus lens in the optical axis direction, and setting a lens position having the highest evaluated value as a focusing position.
Here, the focus lens refers to a lens that moves in the optical axis direction to adjust the focal distance of a photographing optical system. In a lens unit including a plurality of lenses, the focus lens indicates a lens that adjusts a focal position. In the case where the entire group of lenses extends, the focus lens indicates all the entire group.
There are various methods for determining a lens position having the maximum evaluated value. For example, Patent Literature 1 (JP-A-2005-345695) discloses a method for calculating a lens position (focusing position) corresponding to a maximal point of an evaluated value curve by performing a so-called three-point interpolation calculation with respect to three points (a plotted point P2 of the maximum value (the maximum evaluated value) among obtained evaluated values, a plotted point P1 of an evaluated value obtained before the maximum evaluated value, and a plotted point P3 of an evaluated value obtained after the maximum evaluated value) in a graph in which the horizontal axis represents a lens position and the vertical axis represents an evaluated value. Specifically, the focusing position is obtained from an intersection between a straight line that passes through the point P2 and the point P3 and has a gradient of a and a straight line that passes through the point P1 and has a gradient of —α.
In addition, Patent Literature 2 (2004-279721) discloses a method for determining a function of a curve passing through the three points and calculating a lens position corresponding to a maximal point of an evaluated value curve from the function. A spline function or a Bezier function is used as the function.